Conventional prior art devices that are configured to protect or defend the ear against excessive noise can be divided into three (3) major classes; namely, ear plugs, ear muffs and semi-inserts. There are a number of differences between each of these device classes. More specifically, ear plugs are inserted into the ear canal while ear muffs cover the entire outer ear and are held in place by appropriate support means, such as a head band. Semi-inserts are held against the entrance of the ear canal by means similar to those used for ear muffs. Each class of device also has its own advantages and disadvantages.
In this regard, ear plugs can provide substantial noise reduction (attenuation) distributed reasonably evenly over the audible frequency range of sound. In addition, these devices are small and inexpensive. However, ear plugs may cause discomfort due to their required positioning in the ear canal. These devices may also cause hygiene problems resulting from cerumen (ear wax) and the possibility of introducing harmful substances into the sensitive ear canal that may produce irritation as well as infection of the ear canal walls. Further, there is the added problem of fitting ear plugs to individual ear canals, which vary in size and shape over a fairly wide range.
Ear muffs may provide the highest noise attenuation at sound frequencies exceeding 1000 Hz, but these latter devices are generally inferior to ear plugs at lower frequencies. Although these frequencies appear to be less damaging for the ear, they produce relatively strong masking of useful signals, such as speech, and can have a fatiguing effect. Because of their size, conventional ear muffs are cumbersome and can become uncomfortable when worn for prolonged periods of time. That is, these devices further exert pressure on the sides of the head when worn and additionally can produce a bothersome warming effect to the wearer. Additionally, ear muffs are significantly more expensive than either ear plugs or semi-insert devices.
Semi-inserts reduce the problems of individual fit and hygiene found in ear plugs and are less cumbersome and less expensive than ear muffs. Several prior art versions of these devices however have provided less noise attenuation. In an effort to improve the effectiveness of semi-insert devices in terms of noise (attenuation) reduction, Applicants have previously developed a passive semi-insert ear defending device, as described in U.S. Pat. No. 5,824,967, and referred to herein as an “ear muffler”.
As shown in FIG. 1 and described in the '967 patent, an exemplary ear muffler 10 comprises a cylindrical hollow muffler tube 12 closed at one end using a cap 14 and connected to the ear canal (not shown) through an extension 16 and connecting tube 18 of a reduced diameter that enables an adequate fit with the outer ear. The end of the connecting tube 18 defines an opening 28 to provide communication with the ear canal. A tight fit of the connecting tube 18 with the ear canal is achieved by means of a soft foam sealing cuff 20, which is fastened to the connecting tube 18. The muffler tube 12 is filled with a sound absorbing material 22 such as cotton, felt or nylon fibers, and is held in place on the head of the wearer (not shown) by means of a springy headband 24, the headband 24 being adjustably mounted on the tube 12 with the help of a holder 26, the latter being firmly attached to the muffler tube 12. To achieve a tight seal around the ear canal entrance, the springy headband 24 is made to gently press the connecting tube 18 against the ear, so as to provide a compressive force for the connecting tube 18 and more specifically the soft sealing cuff 20 protruding beyond the connecting tube 18. Only one half of the ear muffler 10 is shown in detail in FIG. 1, with the remaining half being a mirror image and also having a muffler tube, extension and connecting tube, each configured to be disposed symmetrically on the other side of the head of the wearer.
In terms of the operation of the device 10, the noise reduction (sound attenuation) that is achieved can be described by the relation Pa/Pe=As=(Ze+Zs)/Ze, in which Pe is the sound pressure at the entrance of the ear canal, Pa is the sound pressure in ambient air, As is the sound attenuation, Ze is the acoustic impedance at the entrance of the ear canal and Zs is the acoustic impedance of the seal provided by the semi-insert through which sound must penetrate into the ear canal. If Zs is much larger than Ze, as it should be in effective devices, the above formula can be approximated by As=Zs/Ze showing that the sound (noise) attenuation is directly proportional to the acoustic impedance at the entrance of the ear canal. Using a muffler tube, such as that described according to FIG. 1, the noise attenuation is increased by placing a small acoustic impedance Zm, in parallel with the impedance of the ear canal Ze. When Zm is sufficiently small, the sound attenuation approaches Asm=Zs/Zm and the improvement in sound attenuation becomes Am/As=(Ze/Zm). In the limit, the improvement in sound attenuation is proportional to the ratio between the ear canal and the shunting impedances.
The muffler tube 12 together with its hollow extension 16 measures about 13 cm in length and provides a quarter wave resonance around 650 Hz, which further increases the ambient noise reduction in the broad vicinity of this frequency. Additional details relating to this sound defending device are provided in the '967 patent, referenced above, the entire contents of which are herein incorporated by reference.
Though the passive semi-insert device described by the '967 patent is a significant improvement over other previously known semi insert devices, it has been later determined that further improvements can be made. For example, the soft sealing cuff extends significantly outward from the end of the connecting tube and is prone to vibrations and also can create a potential source of noise leakage. In addition, there are additional features that can be added to further tune or suitably adjust damping characteristics of the device.